(1) Field of the Invention
The invention relates to electrical power supplies, and more particularly to sensing the current through coils of switching regulators as e.g. boost or buck converters.
(2) Description of the Prior Art
Integrated switching regulators as e.g. boost or buck converters usually have a current control loop requiring accurate sensing of the actual current through an external coil during operation. Usually a small resistor is therefore connected in series to a large MOSFET switch, which conducts this current during on state. FIG. 1 prior art shows a schematic of such a conventional prior art configuration using an NMOS switch. The circuit of FIG. 1 comprises an external coil 1, a large MOSFET switch M1, a sense resistor R, a diode 2, and a capacitor CL and a resistor RL, both representing a load. The part 3 on the left side represents the inductor and the current sense function of the integrated switching converter. The current I0 flows through the coil 1, the MOSFET switch M1, and through the resistor R during on state of the MOSFET switch M1.
Using resistor R for current I0 sensing introduces inaccuracies, especially when integrated on chip, and reduces efficiency. As a consequence a larger pass-device M1 area is necessary, too
There are patents known dealing with the current sensing of switching converters:
U.S. Pat. No. 6,731,524 to Elek et al. describes a power system including a plurality of power supply units (“PSUs”), each PSU having an output that is coupled to the output of other PSUs in the power system. Each PSU includes a power factor correction (“PFC”) assembly for receiving an AC input and generating a first DC output. A DC/DC converter assembly is coupled to the PFC assembly, and receives the first DC output and generates a second regulated DC output. A control assembly is coupled to the DC/DC converter assembly, and is operative to monitor the DC/DC converter assembly output and in response thereto provide control signals to the DC/DC converter assembly. A preferred current sense circuit comprises a thevenin resistor that is coupled into the output path of the DC/DC converter and associated circuitry that generates a current measurement that is proportional to the output current flowing through the DC/DC converter.
U.S. Pat. No. 6,504,351 to Eagar et al. discloses systems and methods for reducing a reverse recovery current through a body diode in a synchronous switching transistor. An inductor is coupled in the commutation path of the body diode of the synchronous switching transistor. The inductor slows the rate of increase of the reverse recovery current to reduce avalanche effects in the synchronous switching transistor. This reduces the peak reverse recovery current through the body diode of the synchronous switching transistor when the body diode commutates, thereby reducing power dissipation in the main switching transistor. An inductor may be coupled to both switching transistors so that power dissipation is reduced if the regulator is operated as a buck or boost regulator. A diode and a reverse recovery switcher may be coupled to the inductor to transfer energy in the inductor back to the input or output capacitor after the body diode commutates. A current comparator monitors the current through the sense resistor in a current mode controller.
U.S. Pat. No. 5,532,918 to Mayrand et al. discloses a high-power-factor power supply having a full-wave rectifier for rectifying an AC line voltage, a power regulator including switch means responsive to a control signal for controlling the application of the rectifier output to a load; and a control circuit for producing a switching control signal. The control signal includes a pair of AC line detectors: a first connected in a closed-loop automatic gain control arrangement, and the other connected in an open-loop arrangement. The control circuit initially produces a current demand reference signal that is directly related to the difference between the power supply DC output voltage and a self-generated constant reference, and to the waveform shape of the AC line voltage, and is inversely related to magnitude changes of the AC line voltage. The control signal then produces the switching control signal in response to both the current demand reference signal and the current flowing in the power supply. The invention avoids the square law dependency of prior art high-power-factor power supplies, can operate over a broad input dynamic range, and is “self-biasing” in that the power supply itself generates any needed reference voltages. In response to a current sense demand reference and a current sense signal from a power regulator indicative of the current flowing through a sensor resistor, a control circuit generates the switching control signal.
Furthermore Philip E. Allen and Douglas R. Holberg teach in “CMOS Analog Circuit Design”, Oxford University press 1987, on page 248 a current loop to create a current out of a reference voltage, here a base-emitter voltage drop and a resistor.